BUDDING TRANSITIONS OF FLUID-BILAYER VESICLES - THE EFFECT OF AREA-DIFFERENCE ELASTICITY

Budding and vesiculation are prominent shape transformations of fluid lipid-bilayer vesicles. We discuss these transitions within the contex t of a curvature model which contains two types of bending energy. In addition to the usual local curvature elasticity kappa, we include the effect of a relative areal stretching of the two monolayers. This are a-difference elasticity leads to an effective nonlocal curvature energ y characterized by another parameter kappaBAR. We argue that the two c ontributions to the curvature energy are typically comparable in magni tude. The model interpolates smoothly between the spontaneous-curvatur e model (kappaBAR = 0) and the bilayer-couple model (kappaBAR --> infi nity), discussed previously in the literature. Conceptually, this mode l is not new; however, neither its consequences nor its relation to ex periment has previously been explored in detail. In particular, buddin g is discontinuous (first order) for small kappaBAR but changes via a tricritical point to continuous (second order) for large kappaBAR. The order of the budding transition depends on both the ratio kappaBAR/ka ppa (which is a material parameter) and the initial area difference be tween the inner and outer monolayers (which can be modified by appropr iate treatment of the vesicle). Estimates suggest that, under typical laboratory conditions, the budding process should be discontinuous, in apparent disagreement with some recent experiments. Possible reasons for this discrepancy are discussed. We propose, in particular, that hy steretic effects are important and that the observed behavior may refl ect a spinodal instability.